In the manufacture of microelectronic devices and circuits, there exist a number of applications in which discrete devices that have been manufactured separately from one another must be brought together in electrical contact. For example, integrated circuits (or "chips") often need to be mounted to printed wiring boards, printed circuit boards or any other such devices, most of which can be generally referred to as "chip carriers." The contact between the chip and the chip carrier must have physical, chemical and electrical integrity and stability.
One typical method of forming semiconductor devices has included the fabrication of metal bumps on the top or exposed surface of various substrates. In recent years, greater interest has been observed in such methods. One major type of metal bumps are those formed of solder; i.e., a low melting alloy, usually of the lead-tin type, used for joining metals at temperatures below 700.degree. F. The bumps are brought into contact with a metal structural element often referred to as a "metallurgy" typically metal pad--that will wet with solder, after which heat is applied to join the solder bump and the metal pad and thereby form the electrical connection.
For example, the metallization layers on semiconductor devices, which are often referred to as "contact pads," are typically formed of aluminum, or in some other applications, copper. Under many circumstances, however, aluminum is sensitive to the subsequent physical, chemical and electrical processes used to fabricate the device after the aluminum has been applied. Aluminum also joins rather poorly with most solders. In order to protect the aluminum during further processing and to facilitate the addition of solder, barrier metals are added to exposed portions of the aluminum. An appropriate barrier metal will be compatible with both the aluminum contact pad and the solder and will form an appropriate transition between them.
In typical applications, solder is first deposited on the contact pad or barrier metals and then remelted. The remelting encourages the liquified solder to retreat from the areas of the substrate which are not wettable by solder, specifically the passivation layer. As the solder retreats, its inherent surface tension draws it into droplets which, because of their generally spheroid shape, extend above the remainder of the semiconductor and form appropriate solder bumps. The techniques of reheating the solder in order to encourage it to form droplets is referred to as a "wet-back" or "reflow" of the solder, and the barrier metal layers under the solder bumps are referred to as the "under bump metallurgy."
When the integrated circuit or other device carrying the bumps is combined with a chip carrier, the solder bumps form contact points which can be joined to appropriate pads often using pressure and heat. A number of processes and resulting structures for solder bumps are currently used. A method used by IBM is the "controlled collapsed chip connection" ("C-4") technique. In this process, a metal mask is aligned on the semiconductor material, a barrier metal alloy which includes gold, copper and chromium is deposited, a second mask is aligned, and then solder is deposited and then reheated to wet back and form droplets. In other processes, such barrier metals are first deposited, then masked with one of the typical photosensitive materials known as photoresists, electroplated with solder, then etched one or more times and then wet back.
There are a number of problems associated with such techniques, however. Some of the processes require rather large capital investment in sophisticated equipment and tools. Other techniques are limited to specific substrates, specific under bump metallurgies, or specific solder systems, or specific combinations thereof. Still other techniques fail to repeatedly produce solder bumps of a consistent defined height, an obvious problem whenever proper physical and electrical contact must be produced and maintained on the scale of an integrated circuit.
Finally, other problems arise as the wafers upon which semiconductor devices are formed become larger and larger. Because of the temperatures at which certain processes take place, and the differences in the coefficients of thermal expansion between wafers of substrate material and the metal masks required to pattern and deposit under bump metallurgies and solders, the masks and wafers can easily become misaligned during these steps, causing unacceptable defects in the resulting device products.